Sngrep - SIP Packet Capture and Analysis Tool - cmd Cheatsheets

Understanding Sngrep: A SIP Packet Analysis Tool

Sngrep is a powerful command-line utility designed for capturing and analyzing Session Initiation Protocol (SIP) packets. It provides a user-friendly, interactive interface within the terminal, making it an invaluable tool for network administrators, VoIP engineers, and developers who need to troubleshoot SIP-based communication systems. By visualizing SIP call flows and message details, sngrep simplifies the process of debugging and understanding complex call setups and signaling.

Core Functionality and Usage

The primary function of sngrep is to intercept and display SIP traffic in real-time. It can operate on all available network interfaces or be directed to a specific one. The tool also supports saving captured packets to a file for offline analysis and loading previously saved captures. This flexibility allows for both immediate troubleshooting and in-depth post-mortem investigations of SIP communication issues.

Basic SIP Packet Capture

To start capturing SIP packets on all network interfaces, simply run:

sngrep

Capturing on a Specific Interface

If you need to monitor traffic on a particular network interface, such as eth0, use the -I flag:

sngrep -I eth0

Saving and Loading Captures

To save the captured SIP packets to a file named capture_file.sng for later review:

sngrep -w capture_file.sng

To analyze a previously saved capture file:

sngrep -r capture_file.sng

Advanced Filtering and Options

Sngrep offers various options to filter and refine the captured SIP traffic, enabling users to focus on specific aspects of the communication. These filters are crucial for isolating problems in busy network environments.

Filtering by SIP Method

You can filter packets to display only those related to a specific SIP method, such as INVITE:

sngrep method INVITE

Excluding Specific Hosts

To exclude traffic originating from or destined for a particular IP address, like 192.168.1.10:

sngrep -X 192.168.1.10

Specifying Capture Port

If your SIP traffic uses a non-standard port, you can specify it using the -d flag. For example, to capture on port 5060:

sngrep -d 5060

Using Berkeley Packet Filters (BPF)

For more complex filtering needs, sngrep supports Berkeley Packet Filters (BPF) via the -f option. For instance, to capture traffic related to a specific host 192.168.1.20:

sngrep -f 'host 192.168.1.20'

Terminal Display Customization

Sngrep allows for some customization of its terminal output, including color schemes. To apply a different color scheme (e.g., scheme 5):

sngrep --color 5

External Resources for SIP Analysis

RFC 3261 - SIP: Session Initiation Protocol: The foundational document for SIP.
MDN Web Docs: MIME Types: Understanding MIME types is often relevant for SIP message payloads.
Wireshark SIP Protocol Reference: While sngrep is terminal-based, Wireshark's documentation provides deep insights into SIP packet structure.
Wikipedia: Session Initiation Protocol: A good overview of SIP and its role in VoIP.